Posts Tagged ‘contaminants’

Last week we identified some inefficiencies in our water to steam power plant energy cycle. The superheater addressed some of these concerns, but not others. Our illustration discloses one of these wasteful areas to be coming from the turbine exhaust. That’s energy laden steam being expelled into the surrounding atmosphere! It’s the same heat energy that was produced in the boiler when water was transformed into steam. It came from burning fuels like coal, natural gas, and oil, all expensive and precious natural resources.

In its present configuration the power plant will work, but because steam is being continually dispersed into the atmosphere, it must continually be replenished. The key ingredient, water, must be drawn into the power plant from a nearby source, treated for contaminants, then fed into the boiler to make up for lost steam. That wastes both water and energy, because the make-up pump, which draws water from the lake for treatment, (thus “making up” for spent water), is continuously operating, resulting in excessive wear and tear and increased operating costs.

Fortunately, power plant engineers have devised methods to correct these inefficiencies. They’ve come up with a clever means of recapturing exhaust steam, thus enabling it to recycle within the system. Next week we’ll see how this is accomplished with a piece of equipment called a condenser.

Last time we learned that electric utility power plants must have water treatment systems in place to remove contaminants from incoming feed water before it can be used. This clarified water is then fed to a boiler by the boiler feed pump as shown below.

As it stands this setup will work to provide electricity, however in this state it’s both inefficient and wasteful. We’ll see why in a minute.

Boilers, as their name implies, do a great job of heating water to boiling point to produce steam. They do this by adding the heat energy produced by burning fuel, such as coal, to water, then steam. We learned in earlier blogs in this series that the energy used to heat water to boiling point temperature is known as sensible heat, whereas the heat energy used to produce steam is known as latentheat. The key distinction between these two phases is that during sensible heating there is a rise in temperature, during latent heating there is not. For a review on this, see this blog article.

When water starts to heat inside the boiler, sensible heat energy is said to be added. This is represented by phase A of the graph below.

During A, heat energy will raise the temperature of the water to boiling point. As the water continues to boil in phase B, water is transforming into steam. During this phase latent heat energy is said to be added, and the temperature will remain at boiling point.

In phase C something new takes place. The temperature rises beyond boiling point and only steam is present. This is known as superheated steam. For example, if the boiler pressure is at 1,500 pounds per square inch, steam becomes superheated at temperatures greater than 600°F.

Unfortunately, boilers alone do a poor job of superheating steam, that is, continuing to raise the temperature of the steam present in phase C. This is evident by the fact that phase C is quite small in comparison to phases A and B before it. This inefficiency in producing ample amounts of superheated steam results in a small amount of useful energy being provided to the turbine down the line, which is bad, because steam turbines require exclusively superheated steam to run the generator.

Next time we’ll see how to provide our steam turbine with more of what it needs to run the generator, more superheated steam.

Ever venture into your basement and stare in amazement at the labyrinth of ductwork stemming off from the furnace? Believe it or not, there’s a science behind that spaghetti bowl configuration. Ductwork can either be flexible or rigid, square or round in shape. Its job in a local exhaust ventilation system is to carry airborne contaminants from the originating source, the carefully positioned hood in the workplace, to the exhaust stack where it is vented outside the building. This job isn’t an easy one. Fluids, like air thick with toxins and toxic gases, don’t want to flow very well through ductwork unless you make their path as unimpeded as possible.

You can think of the air and contaminants flowing through ductwork as if it were like a car moving down a highway. Expressways don’t have sharp 90 degree turns or abrupt changes in width. These would cause a slow down in traffic, unless of course an accident is in the way. Expressways also tend to be rather large thoroughfares. The science behind ductwork follows the same basic principles to work effectively. It will minimize or eliminate sharp turns and it will avoid any abrupt changes in diameter. It will also be as wide in diameter as the environment will accommodate in order to move air volume most effectively.

A local exhaust ventilation system’s performance can be greatly hampered and workers exposed to hazards if ductwork leaks. And if the leaks are upstream of the fan and large enough, they can reduce the ability of the local exhaust ventilation system to draw the airborne contaminants into the hood. Air starts getting drawn in through the leaks rather than through the hood. If the leaks are downstream of the fan, the airborne contaminants can re-enter the work area through the leaks rather than going outside through the exhaust stack.

Ducts come in an endless variety of diameters, the diameter being part of a simple mathematical equation relating to flow velocity. In the simplest of terms, the flow of air through ductwork is governed by the following equation:

Q = V × A

where Q is the flow rate of air through the duct in cubic feet per minute (CFM), V is velocity of the air flow in feet per minute, and A is the cross sectional area of the duct in square feet.

As an example, suppose you want to design a local exhaust ventilation system with ducts no greater than 5 inches in diameter because of space and clearance limitations. You want to use round ducts for the system because they handle air more efficiently and have no sharp corners where dust can collect. If an industrial hygienist determines that the air is required to flow at a minimum of 800 feet per minute through the duct, what is the airflow rate through the duct? Well, since we are dealing with a round duct, its cross sectional area is that of a circle:

A = (π × d2) ÷ 4

where d is the diameter of the inside of the duct as shown in Figure 1 below.

Figure 1 – Cross Section of a Round Duct

So to use this equation for area, to solve for Q, then we must first convert the duct diameter from inches to feet, which makes our equation look like this:

5 inches ÷ 12 inches per foot = 0.416 feet

This gives us a duct cross sectional area of:

A = (π × (0.416 feet) 2) ÷ 4 = 0.136 square feet

And the air will flow through the duct at this rate:

Q = V × A = 800 ft./minute × 0.136 ft.2 = 108.8 CFM

This air flow rate is good to know, because it will help the designer to select an appropriate fan for the local exhaust ventilation system. This is because fans are listed in manufacturers’ catalogs according to how many CFM they can handle.

Next time, we’ll learn more about the rest of the local exhaust ventilation system, namely, the filter, fan, and exhaust stack.